In the very-high-speed/very-long-distance optical transmission systems by an Intensity Modulation/Direct Detection (IM/DD) method, an error correction code has been mainly used as means for improvement of reception sensitivity of the optical receiver. However, in the recent rapid increase in a transmission volume and a transmission distance, application of a technology for further improvement of reception sensitivity of the optical receiver is indispensable. There is a demand for a simple technology for improving reception sensitivity other than the error correction code.
Conventionally, as the technology for improving reception sensitivity of this type, there is an example disclosed in a patent document 1 described below.
In the patent document 1, improvement of reception sensitivity is attained by providing k sets (n×k) discriminators that have different thresholds from one another with n discriminators having an identical threshold as one set, an optical distributor that distributes a reception signal to the respective discriminators, and a selector that selects one of discrimination results of the respective discriminators and outputs the discrimination result, performing majority judgment using the selector, and selecting a discrimination result with higher accuracy from the discrimination results of the respective discriminators.
In a nonpatent literature 1 described below, for example, as an application of the reception technology described in the patent document 1, improvement of sensitivity of a receiver is realized by subjecting one optical signal to wavelength division at a reception stage, providing discriminators for the respective optical signals divided, and adopting more probable data from discrimination results in the respective discriminators.
Patent Document 1
Japanese Patent Application Laid-Open No. 2003-234699
Nonpatent Literature 1
Masahito Tomizawa, Yoshiaki Kisaka, Akira Hirano, and Yutaka Miyamoto, “Error correction without additional redundancy by novel optical receiver with diverse detection,” in OSA Trends in Optics and Photonics (TOPS) vol. 70, Optical Fiber Communication Conference, Technical Digest, Post-conference Edition (Optical Society of America, Washington D.C., 2002), WX7, pp. 368-370.
In the realization of the very-high-speed/very-long-distance optical transmission systems, improvement of sensitivity of the optical receiver is an important element and a technology that is simple and has an easy configuration is indispensable.
However, the invention described in the nonpatent document 1 uses a method of dividing a spectrum of an optical signal using an optical filter. Thus, the invention has a problem in that a division penalty in the optical filter tends to occur and sufficient improvement of sensitivity cannot be expected.
Since the invention also uses the optical filter in implementation, there is also a problem in that limitation on a configuration occurs in terms of dependency on a signal modulation system, stability with respect to temperature, and the like.
Moreover, even if improvement of sensitivity of the optical receiver is realized by a minimum number of discriminators, it is necessary to control a discrimination threshold by preparing two sets of two discriminators (four discriminators in total). Thus, the method is not a simple and easy method from the viewpoint of control of the discriminators or implementation.
In view of such circumstances, it is an object of the present invention to provide an optical receiver that prevents occurrence of a division penalty due to an optical divider, eliminates dependency on a modulation system, and secures easiness of implementation.